1. Field of the Invention
Embodiments of the present invention are directed to a chromatic diffractive optical element (DOE) corrector. More particularly, the present invention is directed to a DOE corrector for use with multiple wavelengths, associated systems and associated methods.
2. Description of Related Art
Numerous applications require a single objective lens to be used for multiple wavelengths, i.e., at least two wavelengths. This requires chromatic aberrations in the objective lens to be corrected to an acceptable level.
For example, to realize higher optical storage capacity, reduction in beam size is needed. Such reduction may be achieved using a shorter wavelength with a higher numerical aperture objective. However, the shorter the wavelength, the higher the energy of the light. Thus, some traditional media may be damaged using a shorter wavelength. Therefore, to function with traditional media, wavelengths for which that particular medium was designed may still be needed.
A particular application is to high-density digital video disc (DVD) systems that are to remain backwards compatible with DVD and compact disc (CD) formats. In such a system, numerous differences in optical requirements are to be addressed between these formats. For example, all three formats have different wavelengths, different numerical apertures (NAs), different diffraction limited spot sizes, different working distances, and different media thicknesses.
The HD-DVD systems may use blue light, e.g., about 380 nm to about 420 nm, have a numerical aperture of about 0.85, a diffraction limited spot size of about 0.58 μm, a working distance of greater than about 0.3 mm, and a media thickness of about 0.0875 mm. The DVD systems may use red light, e.g., about 630 nm to about 680 nm, have a numerical aperture of about 0.6, a diffraction limited spot size of about 1.32 μm, a working distance of greater than about 0.4 mm, and a media thickness of about 0.6 mm. The CD systems may use infrared (IR) light, e.g., about 780 nm to about 820 nm, have a numerical aperture of about 0.45, a diffraction limited spot size of about 2.11 μm, a working distance of greater than about 0.5 mm, and a media thickness of about 1.2 mm.
One conventional solution includes using one surface having a first phase function providing a high first order efficiency for red and a second phase function providing a high first order efficiency for IR, while providing high zeroth order efficiency for blue. In order to achieve this, a thick DOE needs to be used. For example, to make phase levels that are multiples of 2π for the blue wavelength, the phase delay for a transmission DOE is given by:2π(n−1)d/λ  (1)where n is the index of refraction of the DOE for blue light, d is the thickness of the DOE and lambda is the wavelength of the blue light. The 2π thickness D for each wavelength and corresponding refractive index is given by:D=λ/(n−1)  (2)
Thus, for example, if a DOE is designed to transmit 407 nm (blue light), impart the first phase function on 650 nm (red light) and impart the second phase function on 785 nm (IR), since 785 nm is nearly twice 407 nm, levels which effect 785 nm but would not effect 407 nm need to be determined. The phase levels would be determined from integer multiples M of D that do not effect the blue light. For most materials this results in very thick elements with relatively low efficiency, especially in the IR, e.g., less than 50%.
In this current solution using one surface to diffract two of the three wavelengths, phase levels for a first phase function at a first wavelength, e.g., 650 nm, are selected that correspond to a zero phase delay (modulo 2π) or about zero phase delay for the other two wavelengths, e.g., 405 nm and 785 nm. For a second phase function at a second wavelength, e.g., 785 nm, phase levels are chosen to correspond to zero for the other two wavelengths, e.g., 405 nm and 650 nm. Assume the phase levels are provided in a material having no dispersion and a refractive index of 1.46. For simplification, consider only solutions MD for blue light. In designing the second phase function and restricting the multiple of D to M≦40, and then looking for values of M within this range where the phase angle for the red light is less than ±20°, then there are five values for M which satisfy this condition. However, these phase levels also need to provide phase angles close to 0°, 90°, 180° and 270° for a four phase level diffractive for the IR light. Only three of the five values are within ±20° of these target values. A diffractive other than a binary diffractive would thus need to be made with more than a thickness of M=40 at 407 nm, i.e., more than 35 microns thick.
The actual problem is even more severe than in this simplified case, since the refractive index of fused silica actually decreases as wavelength increases, i.e., positive dispersion. Thus, the refractive index of fused silica is actually 1.470 at 405 nm, 1.457 at 650 nm, and 1.453 (at 785 nm). This dispersion results in the blue and IR light becoming even more closely harmonic, as can be seen with reference to the following phase delay ratio of Equation (3):
                                          λ            B                    /                      (                                          n                B                            -              1                        )                                                λ            IR                    /                      (                                          n                IR                            -              1                        )                                              (        3        )            Without dispersion, i.e., when nB=nIR, this phase delay ratio is 1.93, while in fused silica, it becomes 2.01. With these refractive indices, when M is selected to be an integer for the blue light, then phase values for the IR light will all be within ±10° of either 0° or 180° for all values of M<75, resulting in a DOE having a thickness of at least 65 microns to realize even a four level DOE.
Thus, when using fused silica, the conventional approach is limited to a binary DOE for IR light, unless a very thick diffractive structure, e.g., much thicker than 65 microns, is used. Such a binary DOE has very low efficiency, roughly 40%, compared with roughly 80% for a four-level DOE. Thicker DOEs have numerous problems, e.g., they are more difficult to fabricate, are more sensitive to changes in wavelength, and have performance issues, e.g., shadowing due to the relative aspect ratios of the etch depth and the period.